Thermocouples are a type of temperature sensor used in a large number of applications. Usually, a thermocouple is formed of two different elements in the form of thermowires, which are connected with one another at a junction. The principle, on which a thermocouple works, is that a voltage or electromotive force occurs at the junction (referred to as the hot junction) of the two different elements. This junction is, in such case, arranged in the vicinity of a measuring point, whose temperature is to be measured, or at least the junction is in thermal contact therewith.
The thermowires are for reasons of electrical insulation and also for protection against disturbing external influences embedded in a material, which serves for electrical insulation between the thermowires and/or against the environment of the thermowires.
With time, thermocouples can experience changes in composition due to operational loadings. As a result of these changes, the measured thermovoltage no longer corresponds to the actual temperature. Changes in the composition of a thermocouple are referred to, in general, as deterioration. This deterioration can lead to inexact temperature measurements.
Sometimes when a thermocouple has suffered deterioration, an operator of a temperature dependent process, in which the thermocouple is being used, is, in given cases, not able to detect the deterioration. Other times, the operator can figure out from experience that the temperature measurement is outside of a normal temperature range, in which the process is normally operated and so conclude that a deterioration of the thermowires has occurred. Due to aging processes during use, the thermoelectric homogeneity can, however, be greatly lessened (temperature measurement errors of 1 K to 100 K are possible). However, the operator is, in some cases, not able to detect that a thermocouple has weakened, since varied sources of error can be present. Furthermore, the operator is, as a rule, not able to detect the beginning phase of the deterioration of a thermocouple.
Correspondingly, it is known from the state of the art, such as, for example, from U.S. Pat. No. 6,344,747 A1, to compare the resistance of various measuring points with one another for detecting deterioration.
Furthermore, it is known to use particular insulation for protection against substances penetrating from the environment, for example, penetration of a component of the measured material.
In such case, the distance, over which the thermowires are led, is decisive for sensor performance and life. Especially is this true in the case of, for example, mineral insulated thermowires. It should, consequently, be avoided as much as possible that the thermowires extend through hot zones, such as occur, for example, in the vicinity of an oven. The failure to meet these measures concerning the distance, along which the thermowires are led, increases the probability of a defective measurement as a result of a so-called virtual junction error—i.e. a parasitic thermovoltage. Thus, substances from the measured material penetrating into the insulation of the thermowires or to the thermowires or a removal of material, from which the thermowires are composed, can lead to a conductive connection between the thermowires lying not at the junction, i.e. the actual measuring point. This short circuit leads to a corrupted measurement signal, since it provides a (parasitic) voltage, which is superimposed on that, which results from the measuring point. For example, in the case of a burner with a combustion chamber spaced from the heating chamber, it can occur that the thermowires are exposed to higher temperatures along the route to a measuring point in the heating chamber.